1. Field of the Invention
Methods and apparatus for producing color television tubes.
2. Prior Art
The present invention relates to devices for exposing the photo-sensitive layer in the photo-chemical manufacturing process for forming the phosphor triple dot raster on the layer of fluorescent material on the tube envelope's faceplate of the screen of a color television picture tube. As is customary, the layer of fluorescent material consists of a mosaic of different kinds of phosphors, emitting as a rule red, green and blue light which are deposited in successive steps on the inside of the tube's envelope, with the respective deposited photo-sensitive layers, by means of at least one optical screen-coating lens whose optically effective surface has a shape so as to simulate a path of the respective electron beam, which are exposed to light at exactly defined points with the aid of a color selecting electrode consisting of a shadow mask and the mask frame.
In prior publications, for example German Published patent application Nos. 1,927,966 and 1,902,538, it is known that, during the operation of color television picture tube the color selecting electrode is heated considerably, owing to its limited transparency, by the electron beams, about 85% of which are trapped or absorbed by the shadow mask sheet, causing it to expand. As a consequence, the position of the mask aperture is not constant in relation to the position of the trios of color phosphor dots associated with the mask apertures on the screen coated with fluorescent material. Thus the electron beams will no longer land exactly on the phosphor triples. This produces a deteriorating effect upon the displayed color picture.
It has been recognized that the color-selecting electrode must be moved in the direction of the longitudinal axis of the color television picture tube toward the screen coated with fluorescent material (phosphor) in order to hit the phosphor triples exactly. In accordance with the teachings of the above referenced German publications this is accomplished by utilizing suspension elements consisting substantially of bimetal springs which, upon being heated, are deformed to such an extent as to shift the color-selecting electrode in the desired way toward the coating of fluorescent material. Accordingly, these conventional suspension elements shift the color selecting electrode in such a way that the effect of their expansion is compensated in the final state.
Meanwhile, however, investigations have proven that during the warm-up phase of the color television picture tube the thin shadow mask, due to its relatively limited transparency, is very rapidly heated up by the impinging electron beams, while on the other hand the mask frame, because of its larger mass, is not heated up as quickly as the shadow mask and takes more time before it reaches its operating temperature. During this time the shadow mask is arched because of the expansion created by the shadow mask being heated up and it is not compensated for by a corresponding expansion of the mask frame. Accordingly, the landing points of the electron beams on the coat of fluorescent material are displaced and the phosphor triples are not hit by the electron beams as exactly as they should be. In order to obtain a true color display in the warm-up operating state the shadow mask must be moved away from the coat of fluorescent material in the direction of the longitudinal axis of the color television picture tube.
The necessary and previously described movement of the color-selecting electrode toward the coat of fluorescent material should not take place until after both the shadow mask and the mask frame have reached almost the same temperature.
According in my prior German patent application No. P 23,59,844.9, I have already proposed to use suspension elements which cause the color selecting electrode to be first of all moved away from the coat of fluorescent material when the color television picture tube is put into operation and only thereafter towards the coat of fluorescent material. These suspension elements consist of two bimetal elements, each of which is composed of two oblong metal strips firmly connected to one another along their narrow longitudinal sides, which have different heat expansion coefficients. The one bimetal element is attached with its one end to the mask frame by spot welding and its other free end the second bimetal is overlappingly attached by a spot weld or a welded seam. At the other end of the second bimetal element an eyelet or opening is provided for attaching the suspension element to the rim extending around the envelope. Both bimetal elements are attached to one another in such a way that each time the metal strip of the one bimetal element having the smaller heat expansion coefficient is connected to the metal strip of the other bimetal element having the larger heat expansion coefficient.